Recovery of liquid hydrocarbons



G. S. BAYS- April 25, 1939.

RECOVRY OF LIQUID HYDROCARBONS FROM WELL FLUIDS Filed Oct. 18, 1938 I nven :"or'` George By L Patented Apr. 25, 1939 UNITED STATES PATENT .OFFICE RECOVERY F LIQUID HYDROCARBONSI FROM WELL FLUIDS poration of Delaware Application October 18, 1938, Serial No. 235,589

11 claims.

This invention relates to the recovery of liquid hydrocarbons from well iluids and more particularly to improved methods and apparatus for economically recoveringv light liquid hydrocarbons from well fluids produced at high pressures with higher total recoveries of these hydrocarbons from the producing formations than has heretofore been possible.

Before the advent of deep drilling, wells were classified as oil or gas wells depending upon whether a substantial amount of crude oil was obtained or whether the only product was natural gas, and it was early recognized that the natural gasfrom diffrent wells varied considerably in wetness, i. e., the content of liqueiiable hydrocarbonsv capable of being recovered therefrom by well-known methods such as cooling, compression, absorption in a liquid medium, adsorption or combinations of these. 'In recent years, however,

20 certain iields have been discovered in which the only products are natural gas and a relatively small amount of a vvolatile hydrocarbon mixture which is called distillate. Some wells have also been foundwhich produce at a high gas-oil ratio 25 relatively small amounts of heavier hydrocarbons such as those usually associated with the term crude oil together with the'more volatile distillate. A

My invention is primarily concerned with dis- 30 tillate wells but can also be vadvantageously applied to wells of the latter type. 'I'hese wells have many characteristics in common, including high pressure and high gas-oil ratio, and for purposes of simplicity both will be referred'to herein as- 35 .distillate wells and the marketable products therefrom as distillates.

The specifications which distillates must meet in order to be readily marketable vary considerably depending upon the uses to which they are 40 to be put and these vspecications may be for example 'those yfor natural gasoline,'motor fuel or pipeline crude oil. In any of these cases those constituents lighter than butane cannot be present in very large amounts and in some cases only 45 a limited amount of butanecan be present in th distillate. `Unstabilized hydrocarbon liquids recovered from well iluids will therefore be referred to herein as condensates to distinguish them from marketable distillates.

50 The reservoirsfrom which distillates are produced are invariably at quite high pressures and temperatures, e. g. 3000-4000 pounds per square inch and 175-225 F., and it is believed that in many cases the hydrocarbons therein exist in 55 asingle phase and that in 'others a portion of (Cl. (i2-175.5)

the heavy constituents is in a separate liquid phase. In any case the composition, pressure and temperature of the well fluids at the well head are usually such that a small amount of condensate is formed and ca be separated at 5 that point. The usual treatment is to reduce the pressure of the well iiuid in one or more stages to a relatively low value, e. g. atmospheric pressure to 200-500 pounds per square inch and separate additional condensate from each stage. 10 This procedure does not give the maximum yield of either total condensate or marketable distillate from the well fluid, and in addition gradually lowers the reservoir pressure to a point at which more and more of the valuable heavier 15 hydrocarbons are precipitated in the pores of the producing formation, and consequently are not recovered.

It would normally be expected that decreasing the pressure on a hydrocarbon mixture would .cause increased vaporization rather than conand vapor phases have identical properties inclding composition, and so are indistinguishable. Below this critical temperature, the reduction of pressure isothermally causes vaporization of any liquid present, the lighter constituents being vapor-ized more rapidly than the heavier ones. 'I'here is, however, a range of temperature above the critical at which a hydrocarbon mixture under suiiiciently high pressure exists in a single phase and isothermal pressure reduction of such a single phase mixture within that range of temperature will cause the gradual separation or condensation of a liquid phase richer in heavy' constituents than the original mixture. This phenomenon, which is known as retrograde condensation, continues until a certain pressure is 40 reached and further isothermal expansion causes normal evaporation of the separated liquid phase. When this procedure is reversed and the hydrocarbon mixture, which is now in the vapor phase,

is subjected to isothermal compression, at iirst a portion of the heavierl constituents condense out as a separate liquid phase until the same pressure is reached, after which the condensed phase vaporizes. 'I'his vaporization is sometimes known as retrograde vaporization but it is the same phenomenonas 4retrograde condensation carried out in the opposite direction. Since this invention relates primarily to therrecovery of condensates by pressure reduction, the phenoml enon will hereinafterbe referred to as retrograde condensation, and the pressure at which the maximum amount of retrograde condensate is obtained will be referred to as the critical retrograde condensation pressure.

From the above it will be seen that the separation of condensate from the well uid from a distillate well by pressure reduction is due to retrograde condensation. It would normally be expected therefore that the best method of recovering distillate from a distillate well would be to reduce the pressure to the point at which the maximum amount of marketable distillate separates out as a liquid phase and withdraw the separated condensate containing this maximum amount of distillate from the residual gas. Because ofthe fact that in the retrograde condensation range the equilibrium liquid phase composition gradually becomes richer in valuable heavy constituents with decreasing pressure, the pressure for maximum distillate recovery is considerably lower, e. g. 10G-300 pounds per square inch lower than the critical retrograde condensation pressure.

I have found that the maximum economic ultimate yield of distillate from a given reservoir requires the use of certain optimum separator conditions above the critical retrograde condensation pressure and reinjection of the residual gas into the reservoir to maintain the pressure therein. By this means I am able to pick up and produce as a part of the well uid any heavy hydrocarbons which may be in the liquid phase in the reservoir as well as to avoid the undesirable loss of such constituents mentioned above.

It is an object of my invention to provide a method of' economically producing valuable liq-` uid hydrocarbons from high pressure wells and simultaneously insuring against lossV of such hydrocarbons in irrecoverable form in the reservoir. Another object is to provide a novel unitary process for the economical production of marketable distillate simultaneously with maximum ultimate recovery from the producing formation. A further object is to provide novel apparatus which is particularly suitable for practicing my improved methods of distillate recovery. Further objects and advantages of my invention will be apparent from the following detailed description read in conjunction with the drawing which represents schematically a suitable apparatus for practicing my invention.

In one of. its broadest aspects my invention comprises the separation of condensate rich in valuable normally liquid constituents under substantially equilibrium conditions from the well fluid from a distillate Well at a temperature about 25-150 F. above the critical temperature for the particular well fluid and a pressure about 100-500 pounds per square inch above the critical retrograde condensation pressure, and compressing the residual gas for reinjection into the producing formation through an input well to maintain the pressure therein. Preferably the Itemperature used is in the lower portion of this range because a larger yield of condensate and of the valuable heavier hydrocarbon in the well iluid is obtained, thereby reducing the amount of residual gas which must be compressed for recycling to the producing formation to obtain a given ultimate yield therefrom. Equilibrium separation in the temperature and pressure ranges specified has the great advantage that excellent yields of marketable distillate are obtained and simultaneously the cost of compressing the residual gas for reinjection is keptat a minimum.

Obviously the actual values for the optimum separator conditions vary considerably with varying well compositions, some typical compositions being given in terms of mol fractions in the following table:

A B C D It is recognized that the critical temperature and critical retrograde condensation pressure of a particular mixture are rather diilicult of -experimental determination. In view of the fact that all of the well fluids to which my invention is applicable contain a very large proportion of methane, .certain temperature and pressure ranges can be stated which will be optimum for a great majority of such well fluids. For example, in most cases I prefer to carry out the equilibrium separation step at a temperature of about 50 to +100 F. and preferably at about 35 to +25 F., and a pressure of about 800 to 1500 pounds per square inch and preferably at about 950 to 1200 pounds per square inch.

My invention also comprises many additional features and these can best be understood from the following specific example. Referring now to the drawing, a producing well l is' shown which is of the distillate type as herein defined. The well .fluid flowing therefrom at the well head will be at a relatively high pressure and temperature, for example, about 200G-4000 or more pounds per square inch and 75-200 F. depending primarily'upon the bottom hole temperature and pressure, the composition and the rate of production. The pressure at the well v head decreases as the rate of flow increases, such pressure drops for example ranging up to 1000 pounds per square inch below the pressure in the subsurface reservoir at high rates of production, and the temperature at the well head is in the neighborhood of 50-l50 F. below the reservoir temperature. In most cases such pressure and temperature drops will causey the formation of condensate as the well fluid flows upward through -the tubing, and it is important that the rate of flow be suiciently high to prevent any liquid flow down the tubing, which would give rise to slugging and erratic pressure and temperature conditions. For purposes of example, it will be assumed that the wellfluid produced by well I has a well head pressure of 3000 pounds per square inch and temperature of 135 F., and that it contains a small amount of liquid condensate.

The welliluid passes through valve 2 which is normally wide open and therefore does not serve to give any substantial pressure reduction and thence through cooler 3. preferably utilizing water as the cooling medium, which lowers the temperature of the well fluid to a temperature above that at which natural lgas hydrates form under these conditions due to the small amount of moisture present therein. At 3000 pounds per square inch this temperature will generally lie in the range 80-90 F., and will be higher at higher pressures. The cooled fluid preferably passes through valve 4 into separator 5 in which the liquid phase entering the base of the well, that formed in the upward passage through the Well and that formed in cooler 3 are separated out, are withdrawn through valve 6 controlled by liquid level controller 1 and pass through line 8 ,to separator 9. The gases from separator 5 pass out through valve I0 and line II and a small amount of antifreeze solution which can, for instance, bea calcium chloride solution or other brine, glycerine, glycol or the like, is injected into line II from line I2 under the control of valve I3. Line I4 by-passing separator 5 is controlled by valve I5, which is kept closed in the type of operation being described'.

,The gases carrying the antifreeze material, the

purpose of which is to avoid clogging of the apparatus by natural gas hydrates, pass through three successive coolers I6, l1 and I8 although it is to be understood that other cooling arrangements can be used. As shownycooler I6. operates by indirect heat exchange with the low tempera.- ture condensate from separator 9, cooler I1 operates by indirect heat exchange with the low temperature gases from separator. 9 and cooler I8 operates by indirect heat exchange with an external cooling medium which can be cold water or brine, or ammonia or other refrigerant supplied by a conventional refrigeration cycle.

With the arrangement as hereinbefore described, and when operating upon a well fluid which is relatively high in moisture content, it isV Possible that there will be enough moisture contained in the liquids collected in separator 5 to cause the formation of gas hydrates in line 8 after the considerable pressure drop which occurs across valve 6. To provide for this contingency, an additional antifreeze line I9 controlled by valve20 is shown for the injection of antifreeze material into'the well fluid before it enters cooler `3.

While reference is made herein to the use of a liquid antifreeze material which can be reconcentrated and recycled as will be described below; it

is not imperative that the prevention of gas hydrate formation be handled in this manner. For

instance, a gaseous antifreeze material can be.

used in the same way, recovered and recycled, or the well fluid can be dried by adsorption, preferably before cooling, to eliminate hydrate-forming tendencies.

The cooled fluids which can suitably be at a temperature of from about 25 F. to about -|-'15 F., or preferably from about 0" F. to about +60 F.,.pass through line 2| and valve 22 and are expanded ,into separator 9. The pressure in separator 9 is controlled by valve 23 leading to line 24 for recompression and ultimate reinjection into the producing formation. 'I'he main pressure drop between separators 5 and 9 is across valves 6 and 22. I

vThe pressure maintained in separator -9 lies within the range of about 100 to 500 pounds perv square inch above the critical retrograde condensation pressure. in accordance with my invention. ,As stated above in most cases the pressure in separator 9 can be from about 800 pounds per square inch to about 1500 pounds .per square inch, and preferably from about 950 to about separator 9 (which should, ofcourse, be suitably insulated) can be from about -50 F. to about F., or preferably from about 35 F. to about +25 F. The temperatures in the lower portion of this range are made possible by the use of antifreeze, and result in improved recoveryV of marketable distillate.

'I'he following example shows the improvement in yield of valuable liquid hydrocarbons obtained by lowering' the temperature of separation under the high pressures specified above: A typical distillate Well fluid (Katy Field, Texas) contains 1.181 gallons of hydrocarbons having more than tity of condensate, and the total condensate together with the antifreeze material settles to the bottom of separator 9, where stratification takes place since the aqueous antifreeze material is substantially hydrocarbon insoluble. The lower layer'of antifreeze 25 is withdrawn through Water leg 26 into trap 21. f

-The upper layer 28 ofA hydrocarbon condensate in separator 9 has its upper surface maintained at a constant level by means of float control 29 operating on valve 30. As the antifreeze accumulates the interface between antifreeze 25 and condensate 28 rises until a point is reached at which the hydrostatic head of liquid in separator 9 is suflicient to carry the antifreeze over `the hump of water leg 26. 'I'he pressures in separator 9 and in trap 21 are maintained equal by means of connecting vapor pipe3l. From trap 21 the antifree'ze passes through valve 32 controlled by liquid level controller 33 into antifreeze reconcentrator 34 which can be of conventional design and which is not shown in detail. The reconcentrated antifreeze passes through pump 35, line I2 and valve I3 back into the `hydrocarbon iluid entering coolers I6 to I8, and also under some conditions through line I9 and valve 20 as stated above. The condensate vpasses through valve 30, line 36 and heat exchanger I6, in which itv serves to lower the temperature of the fluid passing to separator 9.

By passing condensate from separator 5 through line 8 to separator 9, I find that the total yield of distillate containing but/ane and heavier hydrocarbons can be materially increased. Thus, for example, with South Jennings, Louisianawell fluid, separator 5 can lbe operated at well head pressure and temperature, 3140 pounds per square inch and F. (Without using cooler 3), andy separator 9 can be .operated at 1200 pounds per square inch and 0 F. With these conditions the total yield of butanes and heavier separated as liquid is 1.076 gallons per 1000 cubic feet of total propanes and lighter if condensate .is separately Withdrawn from separators 5 and 9, while with the condensate from separator 5 passing to separator 9 to alter the phase conditions existing therein, the total yield is increased to 1.109 gallons per 1000 cubic feet.

While I prefer to use separator 5 to remove condensate from the well fluid at substantially well head pressure and to pass this condensate into separator 8 as described above, since by this means I am able to obtain more contact of gas and liquid and consequently a somewhat closer approach to equilibrium conditions in separator 9. these steps are not essential parts of my invention and may be omitted by -opening valve I5 in separator by-pass line I4 and closing valves 4 and I0. i

After passing through heat exchanger I6, the temperature of the condensate is additionally increased by means of heater 31. Thus, for instance.' this heater can increase the temperature to a value within the range from about 200 F. to about 400 F., for instance 350 F., at about the pressure existing in separator 9 although there is, of course, some slight pressure drop. The purpose of this heating is to drive off a considerable quantity of the methane and ethane contained in the condensate before the pressure thereof is reduced as will hereinafter appear. By using heat to drive on methane and ethane under relatively high pressure conditions, for instance 800 to 1500 pounds per square inch, recompression and stabilization costs are greatly reduced.

From heater 31 the condensate, now containing a considerable amount of vapor phase material, passes into separator 38 where the liquid fraction and the vapors separate. The vapors pass overhead through back pressure control valve 39 while the liquid fraction collects in the bottom of the separator and is withdrawn through valve 40 controlled by liquid level controller 4 l. This fraction can be passed through valve 42 (valves 43 and 44 being closed) directly into stabilizer tower 45, or additional methane and ethane can be removed by iiashing the distillate at a lower pressure prior to introduction'into stabilizer tower 45, and this is usually preferable. It can be accomplished by closing valve 42 and opening valves 43 and 44 whereupon the liquid fraction passes into sepa-- back pressure control valve 4l at some value substantially le'ss than that existing in separator 38, for instance 300 to 500 pounds per square inch. At this pressure the remaining liquid collects at the bottom of the separator 45 and the low pressure gas passes out through control lvalve 4l and line 08, whence it passes either through valve 59 for recompression and ultimate reinjection into the producing formation or through valve 50 for other use.

Valve 00 is controlled by liquid level controller 5i and the liquid removed through this valve or that coming directly from separator 38 passes through heat exchanger 52 where its temperature is raised by -indirect heat exchange with the hot stabilized distillate from stabilizer 05. Supplemental heat is then added by means of heater 53 and the liquid passes into stabilizer tower 'i5 which can be of conventional design and which is preferably operated at from about 200 to about 400 pounds per square inch, for instance about 350 pounds per square inch. Heat is applied to the bottom of the stabilizer by means of heating coil 54 and reflux is provided by means of dephlegmating coil 55. The stabilized distillate passes through valve 5B, heat exchanger 52 and cooler 5l to product storage tank 50.

While I have shown stabilizer 45 as my preferred means of producing a distillate of a quality meeting specifications, the same products can be obtained, although with lower yields, by further stages of expansion and separation in a manner well known in the art, and this procedure is also contemplated as an alternative method of carrying out my invention,

Relatively high pressure gases-from separator 9 pass through line 24, are recompressed by compressor 59 to a pressure substantially higher, e. g.

that of the i'luid leaving producing well l, and then pass through valve 6 0 into input well 6| for reinjection into the producing formation. Similarly the gases at slightly lower pressure from separator 38 pass through line 82, are recompressed by compressor 63 and injected into input well 6I, and, as previously mentioned, the.

separator 46 by means of compressor 64 or they can be withdrawn through valve 50.

Alternatively, of course,v a single set of compressors can be used for all of these gases but since they exist at varying pressures it is preferably to use the arrangement shown.

From the above description it can be seen that I haveprovlded a novel and economical process of recovering marketable distillate from high pressure distillate wells and simultaneously maintaining the reservoir pressure so as to prevent precipitation and loss of valuable heavy constituents in the producing formation, and also to cause any such constituents which are present in the formation in the liquid state to enter the producing well as a part of the well uid.

This is a continuation-in-part of my copending patent application Serial No. 232,401 led September 29, 1938.

While I have described my invention in connection with certain specific embodiments thereof, it is to be understood that these are by way of illustration and not by way of limitation, and I do not mean to be bound thereby but only to the subject matter of .the appended claims.

I claim:

1. The method of recovering liquid hydrocarbons from a fluid produced by a well of the distillate type which comprises expanding said iiuid into a separation stage maintained at a temperature within the range from about 50 to about F. and a pressure within the range from about 800 to about 1500 pounds per square inch, separating the hydrocarbon condensate formed at said temperature and pressure, from the residual gas, heating said condensate to an elevated temperature and passing it to a second separation stage maintained at substantially the same pressure as that in said rst separation stage to remove a gaseous fraction containing substantial quantities of light hydrocarbon gases therefrom, and compressing said residual gas and said gaseous fraction for reinjection into the formation from which said uid was produced.

2. The method of recovering liquid hydrocarbons from a fluid produced by a well of the distillate type which comprises expanding said fluid into a separation stage maintained at a temperature Within the range from about -50 to about -|-100 F. and a pressure within the range from about 800 to about 1500 pounds per square inch, separating the hydrocarbon condensate formed at said temperature and pressure, from the residual gas, heating said condensate to an elevated temperature and passing it to a second separation stage maintained at substantially the same pressure as that in said rst separation stage to 500-'1000 pounds per square inch higher, than remove a gaseous fraction containing substantial quantities of light hydrocarbon gases therefrom. withdrawing the liquid condensate from said second separation stage and removing further quantities of light hydrocarbon gases therefrom at a further reduced pressure to form a marketable distillate product, and compressing said residual gas and said gaseous fraction for reinjection into the formation from which said fluid was produced.

3. The method of recovering liquid hydrocarbons from a uid produced by a well of the distillate type which comprises expanding said fiuid into a separation stage maintained at an elevated pressure in the retrograde condensation range, separating the hydrocarbon condensate formed at said pressure from the residual gas,

, heating said condensate to an elevated temperature and passing it to a second separation stage maintained at substantially the same pressure as that in the first separation stage to remove a gaseous fraction containing substantial quantities of light hydrocarbon gases therefrom, withdrawing the liquid condensate from said second separation stage and removing further quantities of light hydrocarbon gases therefrom at a further reduced pressure to form a marketable distillate product.

4. The method of recovering liquid hydrocarbons from a fluid produced by a well of the distillate type which, comprises expanding said uid into a separation stage maintained at an elevated pressure in the retrograde condensation range, separating the hydrocarbon condensate formed at said pressure from the residual gas, heating said condensate to an elevated temperature and passing it to a second separation stage maintained at substantially the same pressure as that in the first separation stage to remove a gaseous fraction containing substantial quantities of light hydrocarbon gases therefrom, withdrawing the liquid condensate irom said second separation stage, and subjecting said last-mentioned condensate to fractionation at a pressure below that in said second separation stage to stabilize the same and form a marketable distillate product.

5. The method of recovering liquid hydrocarbons from a fluid produced by a well 'of the distillate type which comprises expandingA saiduid into a separation stage maintained at an elevated pressure in the retrograde condensation range, separating the hydrocarbon condensate formed at said pressure from the residual gas, heating hydrocarbon gases therefrom, withdrawing the liquid condensate from said second separation stage, and introducing the same into a third separation stage maintained ata further reduced pressure to remove further quantities of light hydrocarbon gases therefrom, withdrawing the liquid condensate from said third separation stage, and subjecting said last-mentioned condensate to stabilization to form a marketable distillate product.

6. The method of recovering liquid hydrocarbons from a fiuid'produced by a well of the distillate type which comprises separating the liquid fraction from said fluid at about well head pressure and a temperature above that at which natural gas hydrates are formed at said pressure, admixing said fluid after separation of said liquid fraction with an antifreeze material, cooling and expanding the resulting mixture into a second separation stage maintained at an elevated pressure in the retrograde condensation range, introducing said liquid fraction into said second separation stage, withdrawing streams of hydrocarbon condensate, antifreeze material and residual gas from said second separation stage, heating said condensate to an elevated temperature and passing it to a third separation stage maintained at substantially the same pressure as that in said second separation stage to remove a gaseous fraction containing substantial quantities of light hydrocarbon gases therefrom, withdrawing the liquid condensate from said third separation stage and removing further quantities of light hydrocarbon gases therefrom at a further reduced pressure to form a marketable distillate product, and compressing said residual gas and said gaseous fraction from said third separation stage for reinjection into the formation from which said uid was produced.

'1. The method of recovering liquid hydrocarbons according to claim 6 wherein said second separation stage is maintained at a temperature within the range from about 50 to about +100 F. and a pressure within the range from about 800 to about 1500 pounds per square inch, and said third separation stage is maintained at a temperature Within the range from about 200 to about 400 F.

8. The method of recovering liquid hydrocar- I bons according to claim 6 wherein said second separation stage is maintained at a temperature within the range from about 35 to about +25 and a pressure within the range from about 950 to about 1500 pounds per square inch, said third separation stage is maintained at a temperature within the range from about 200 to about 400 F. and light hydrocarbon gases are removed from the condensate from said third separation stage by fractionation at a pressure within the range from about 200 to about 400 pounds per square inch.

9. Apparatus for recovering liquid hydrocarbons from a uid produced by a well of the distillate type comprising a separator, means for cooling and expanding said fluid into said separator,means for withdrawing and heating liquid condensate from said separator, a second -separator, means for introducing said heated condensate into said second separator, means for removing liquid condensate from said second separator, means for removing further quantities of light hydrocarbon gases from said condensate from said second separator, means for removing gases from both of said separators and means for recompressing said last-mentioned gases for reinjection into the formation from which said uid was produced.

10. Apparatus forA recovering liquid hydrocarbons from a fluid vproduced by a well of the distillate 'type comprising a first separator. means for introducing said fluid into said first separator, means for withdrawing and cooling gases from said first separator, a second separator, means for expanding said cooled gases into said second separator, means for introducing liquid from said first separator into said second separator, means for withdrawing and heating liquid condensate from said second separator, a third separator, means for introducing heated liquid condensate into said third separator, means for removing gases from said second and third separators, means associated with said last-mentioned means for maintaining desired pressures within said second and third separators, means for recompressing said gases from said second and third separators for reinJeotion into the formation from which said uid was produced, a stabilizing tower, means for passing liquid condensate from said third separator to said stabilizing tower, and means for withdrawing-stabilized dis mi uuate fromsaid stabmmng tower. 

